1. Field of the Invention
The present invention relates to a method for displaying images such as, for example, radiation images on a displaying apparatus such as a cathode-ray tube (CRT) and more particularly to a method of gradation processing for displaying the images of partial areas of the above described images.
2. Description of the Related Art
Conventionally, radiation images such as, for example, X-ray images have often been used for diagnosing diseases and other diagnostic applications. In the case of X-ray images, for example, X-rays which have passed through a subject are irradiated onto a layer of phosphorescence fluorescent material (fluorescent screen); X-rays are converted to visible rays; a latent image is formed by irradiating these visible rays onto a silver chloride film; and an X-ray image is obtained by developing this silver chloride film. Those X-ray images thus obtained have been used in diagnoses of diseases and other diagnostic applications.
Lately, methods by which digital image information obtained from X-ray CT (computer tomography), MRI and X-ray II camera is displayed on the cathode-ray tube (CRT) and stored in magnetic storage media or the like has been widely utilized. Also in the field of radiation photography where such conventional silver chloride films as described above have been used, those systems capable of digitizing image information and displaying it on the CRT have been proposed. One of these systems is a method which uses an accelerated phosphorescence fluorescent material. A basic system which employs this accelerated phosphorescence fluorescent material is disclosed in detail in the U.S. Pat. No. 3,859,527.
The following paragraphs describe in detail a system which uses the accelerated phosphorescence florescent material.
A phosphorescence fluorescent material used in this system is called an accelerated phosphorescence fluorescent material which accumulates energies of radiations such as X-rays when the X-rays are irradiated thereupon. The state of this accumulation is relatively stable and can be maintained for a long period of time. When a first beam serving as an excitation beam is irradiated-onto the phosphorescence fluorescent material which stores the energies of radiations, an accelerated phosphorescence light with an intensity corresponding to the accumulated energy is radiated as a second beam. In this case, not only visible rays but also those rays with a wide range of wavelength from infrared rays to ultraviolet rays are used. The type of ray, however, differs with the type of fluorescent material to be used. The second beam is variously available in the range from infrared rays to ultraviolet rays. This difference depends on the type of phosphorescence fluorescent material to be used.
An X-ray photography system has been materialized for practical use to obtain radiation image information by irradiating and receiving radiations which have passed through a subject such as a human body onto the above described accelerated phosphorescence fluorescent material while making use of the characteristics of the accelerated phosphorescence fluorescent material.
Specifically, an accelerated phosphorescence light is generated by scanning a plate or sheet made of accelerated phosphorescence fluorescent material, which stores X-ray image information of a subject, with an excitation beam such as a laser beam. This accelerated phosphorescence light is condensed by a photoelectric converter to obtain electrical signals proportional to the intensities of accumulated radiations. Subsequently, the electrical signals are processed for imaging and a visualized radiation image is obtained by printing the image on a silver chloride film or displaying it on the CRT.
FIG. 15 is an approximate structure of the conventional radiation image information reader.
The accelerated phosphorescence fluorescent panel 3-1 on which an X-ray image is accumulated and stored is transferred (sub-scanning) in the Y direction shown with an arrow, by a precision slide 3-7.
During this transfer (sub-scanning), an excitation beam emitted from an excitation beam source 3-4 such as a gas laser, semiconductor laser or the like is repeatedly reflected and deflected by a scanner 3-5 such as, for example, a galvanometer mirror or a polygon mirror and irradiated onto the accelerated phosphorescence fluorescent panel 3-1 after having passed through an optical system 3-6 such as an f.theta. lens for correcting the shape of beam. Thereby the accelerated phosphorescence fluorescent panel 3-1 is repeatedly scanned (main scanning) with the excitation beam in the X direction shown with the arrow. Accelerated phosphorescence fluorescent rays which bear the X-ray image accumulated and are stored on the accelerated phosphorescence fluorescent panel 3-1, are radiated from all scanning points. These accelerated phosphorescence rays are condensed through a light guide passage 3-8 consisting of a plurality of optical fibers which are bound together; guided to a photo-multiplier 3-9 through an optical filter (not shown) which cuts off the excitation beam and admits the accelerated phosphorescence light; and converted into electric signals.
Electric signals obtained from the photo-multiplier 3-9 are amplified by an initial stage amplifier 3-10 to a most suitable signal level for an A/D converter 3-11. Pixel data which are digitized as signals for each pixel by the A/D converter 3-11 is stored in an image memory 3-12. The range of intensity and the gradation curve are converted for display and sent to a video memory 3-12. Pixel data sent to the video memory 3-14 are converted to display luminance signals and displayed on the CRT 3-16 or outputted as a hard copy on a film (not shown).
FIG. 16 illustrates an example of a method for obtaining the conversion information to be recorded on a lookup table 3-13.
First a histogram of pixel values of image data corresponding to a whole image is obtained, the minimum value (Smin) and the maximum value (Smax) to be displayed are determined within the histogram and a lookup table is prepared by correlating the signal levels and the display brightnesses included between these minimum and maximum values in terms of various shapes of curves (so-called gradation curves). Pixel data read out from the image memory 3-12 is converted by using the lookup table 3-13 (refer to FIG. 15) and sent to the video memory 3-14 whereby an image with an appropriate brightness is displayed on the CRT (cathode-ray tube) 3-16.
In this connection, it has been reported that conventional silver chloride films for use in observation of X-ray images of human bodies such as, for example, 14 inch.times.14 inch silver chloride films are provided with a resolution of 4000 dots.times.4000 dots. In contrast, the CRT is usually provided with a resolution of only approximately 1000 dots.times.1000 dots and even an expensive CRT only provides the resolution of 2000 dots.times.2000 dots.
In some cases, in an attempt to rectify such insufficient resolution, the overall image is first displayed to check the presence of an abnormal shade or shades when an image obtained from the above described system is displayed on the CRT for observation and diagnosis. Next, the part of the image, with which the user is concerned, is magnified for further minute observation and diagnosis.
However, the CRT is also inferior to conventional silver chloride films the in the display contrast. For example, silver chloride films can represent the gradation of approximately 12 bits=4096 whereas the CRT represents the gradation of only 8 bits=256.
Conventionally, even for magnifying the display of a part, with which the user is concerned, of the image on the CRT, the conversion information which has been obtained to produce an appropriate brightness over the whole image as described above, has been directly used as the lookup table. Therefore only a partial area of the image is magnified and displayed with its own current brightness whether the partial area has a high brightness when the entire image is taken or a low brightness as a whole. In some cases, accordingly, the magnified image of this partial area has appeared with a low contrast resolution.